As a result of recent demand for a higher capacity and miniaturization of electrical equipment, especially hermetic exothermic equipment such as transformers, motors and converters, wholly aromatic polyamide sheetlike materials called aramid sheets or aramid boards have been used as insulating materials having better heat resistance.
Although cleaved natural mica, which is an inorganic material, has been used in the field when requiring a high level of electrical insulating properties exceeding the durability (especially corona resistance) of ordinary organic electrical insulating materials such as aramid sheets or aramid boards, production of large-sized natural mica of good quality is limited and expensive. Further, the product containing the cleaved natural mica is poor in flexibility. Consequently, there are various problems in molding processability and the like.
Methods for uniformly mixing finely divided mica, a readily available raw material, in place of such natural cleaved mica with a small amount of polymer fibrids, forming a sheet, and hot-pressing the resulting sheet, thereby integrating the mica with the reinforcing material, have been proposed [Japanese Laid-Open (Kokai) Patent Nos. 149703/1982 and 167374/1982]; however, the sheet bonded by the hot-pressing treatment barely develops the necessary strength and is deficient in flexibility. Therefore, the sheet has problems of insufficient responsiveness to complicated demands for processing as electrical and electronic parts.
It has also been proposed to obtain a high-temperature resistant sheetlike structure from an entangled mixture of particulate mica with substantially unfused wholly aromatic polyamide fibrids [Japanese Publication (Kokoku) Patent No. 20421/1968 and corresponding G.B. Patent No. 1129097]. In this method, particulate mica having a relatively small particle diameter is used. However, because of the relatively small particle diameter of the particulate mica to be blended, the particulate mica is not captured sufficiently by the wholly aromatic polyamide fibrids and, thus, easily falls off in hot-compression molding after forming the sheet. Because of this, a problem arises in that dispersion is caused in corona resistance.
Furthermore, there is also a well-known sheet composed of mica particles and wholly aromatic polyamide fibers wherein the mica particles are covered and connected with the wholly aromatic polyamide to provide the so-called mica-including pulp. The purpose of using this mica-including pulp is to provide improvement in oil impregnating properties [Japanese Publication (Kokoku) Patent Nos. 35763/1977 and 7961/1978 corresponding to U.S. Pat. No. 4,060,451]. Such mica-including pulp is excellent in bonding to the wholly aromatic polyamide fibers and improved capture of the fine mica particles can be expected. However, since producing the mica-including pulp requires complicated processes, the pulp is disadvantageous from a practical viewpoint.